The present invention relates to a power switching circuit. More specifically, the present invention relates to technology that can be effectively utilized for a power switching circuit for turning on a vehicle lamp using a power MOSFET (insulated gate field-effect transistor).
The protecting circuit for the overcurrent such as when the load is short-circuited may be of the type having a feedback which operates to turn the output element off when the output current exceeds a predetermined value or of the type wherein the current is limited to a predetermined value. The power switching circuit equipped with such a protecting circuit has been disclosed in, for example, a journal "Electronic Technology" (Smart Power MOS IC), January, 1987, pp. 22-25.
When a lamp mounted on a vehicle is driven by the power switching circuit, the filament exhibits a very small resistance at a moment when the lamp is turned on and a very large current flows. The resistance then increases with the lapse of time as the heat is generated by the filament, and the current that flows decreases. The overcurrent protecting circuit works when such a load is driven. Therefore, the lamp is kept from being turned on by the protecting system which turns the output element off in response to an overcurrent detection signal. In the system which limits the output current to a predetermined value in response to the output overcurrent detection, on the other hand, a given amount of current flows even when the lamp has a small resistance. Therefore, the electric power is consumed in large amounts by the power switching circuit (IC) leaving a problem from the standpoint of protecting the output power element. Moreover, since the current is limited, an extended period of time is required before the lamp is completely turned on.
The object of the present invention is to provide a power switching circuit which consumes reduced amount of electric power during the current-limiting operation.
Another object of the present invention is to provide a power switching circuit adapted to driving the lamp.
The above and other objects as well as novel features of the present invention will become obvious from the description of the specification and the accompanying drawings.
A representative example of the invention disclosed in the present application will now be described briefly. That is, when a current flowing into the power output element is greater than a predetermined value, pulse width-modulated signals are formed that vary in inverse proportion to the current value (magnitude), in order to drive the power output element.
According to the above-mentioned means, the power output element is allowed to intermittently operate for only short periods of time when an excess current flows, and the flow of current can be decreased during the current-limiting operation.